Adds a "high fan-out" indicator that surfaces when a task blocks an unusually large number of downstream tasks, displayed as a warning badge on TaskCards and in the ExecutorStatusBar. The feature includes core type definitions, dashboard UI updates to render the warning with appropriate styling and
Fusion-Task-Id: FN-3942
Implemented the blocker fan-out feature across the dashboard: added a `useBlockerFanout` hook to track blocking relationships, rendered a fan-out badge on task cards, and added a blocking section to the task detail modal, with tests covering the hook and responsive modal behavior.
Fusion-Task-Id: FN-3897
